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Conditional match without false force a match?

I'm using the following regex in c# to match some input cases:
^
(?<entry>[#])?
(?(entry)(?<id>\w+))
(?<value>.*)
$
The options are ignoring pattern whitespaces.
My input looks as follows:
hello
#world
[xxx]
This all can be tested here: DEMO
My problem is that this regex will not match the last line. Why?
What I'm trying to do is to check for an entry character. If it's there I force an identifier by \w+. The rest of the input should be captured in the last group.
This is a simplyfied regex and simplyfied input.
The problem can be fixed if I change the id regex to something like (?(entry)(?<id>\w+)|), (?(entry)(?<id>\w+))? or (?(entry)(?<id>\w+)?).
I try to understand why the conditional group doesn't match as stated in original regex.
I'm firm in regex and know that the regex can be simplyfied to ^(\#(?<id>\w+))?(?<value>.*)$ to match my needs. But the real regex contains two more optional groups:
^
(?<entry>[#])?
(\?\:)?
(\(\?(?:\w+(?:-\w+)?|-\w+)\))?
(?(entry)(?<id>\w+))
(?<value>.*)
$
That's the reason why I'm trying to use a conditional match.
UPDATE 10/12/2018
I tested a little arround it. I found the following regex that should match on every input, even an empty one - but it doesn't:
(?(a)a).*
DEMO
I'm of the opinion that this is a bug in .net regex and reported it to microsoft: See here for more information

There is no error in the regex parser, but in one's usage of the . wildcard specifier. The . specifier will consume all characters, wait for it, except the linefeed character \n. (See Character Classes in Regular Expressions "the any character" .])
If you want your regex to work you need to consume all characters including the linefeed and that can be done by specify the option SingleLine. Which to paraphrase what is said
Singline tells the parser to handle the . to match all characters including the \n.
Why does it still fail when not in singleline mode for the other lines are consumed? That is because the final match actually places the current position at the \n and the only option (as specified is use) is the [.*]; which as we mentioned cannot consume it, hence stops the parser. Also the $ will lock in the operations at this point.
Let me demonstrate what is happening by a tool I have created which illustrates the issue. In the tool the upper left corner is what we see of the example text. Below that is what the parser sees with \r\n characters represented by ↵¶ respectively. Included in that pane is what happens to be matched at the time in yellow boxes enclosing the match. The middle box is the actual pattern and the final right side box shows the match results in detail by listening out the return structures and also showing the white space as mentioned.
Notice the second match (as index 1) has world in group capture id and value as ↵.
I surmise your token processor isn't getting what you want in the proper groups and because one doesn't actually see the successful match of value as the \r, it is overlooked.
Let us turn on Singline and see what happens.
Now everything is consumed, but there is a different problem. :-)

Use OR in Regex Expression

I have a regex to match the following:
somedomain.com/services/something
Basically I need to ensure that /services is present.
The regex I am using and which is working is:
\/services*
But I need to match /services OR /servicos. I tried the following:
(\/services|\/servicos)*
But this shows 24 matches?! https://regex101.com/r/jvB1lr/1
How to create this regex?

The (\/services|\/servicos)* matches 0+ occurrences of /services or /servicos, and that means it can match an empty string anywhere inside the input string.
You can group the alternatives like /(services|servicos) and remove the * quantifier, but for this case, it is much better to use a character class [oe] as the strings only differ in 1 char.
You want to use the following pattern:
/servic[eo]s
See the regex demo
To make sure you match a whole subpart, you may append (?:/|$) at the pattern end, /servic[eo]s(?:/|$).
In C#, you may use Regex.IsMatch with the pattern to see if there is a match in a string:
var isFound = Regex.IsMatch(s, #"/servic[eo]s(?:/|$)");
Note that you do not need to escape / in a .NET regex as it is not a special regex metacharacter.
Pattern details
/ - a /
servic[eo]s - services or servicos
(?:/|$) - / or end of string.

Well the * quantifier means zero or more, so that is the problem. Remove that and it should work fine:
(\/services|\/servicos)
Keep in mind that in your example, you have a typo in the URL so it will correctly not match anything as it stands.
Here is an example with the typo in the URL fixed, so it shows 1 match as expected.

First off you specify C# (really .Net is the library which holds regex not the language) in this post but regex101 in your example is set to PHP. That is providing you with invalid information such as needed to escape a forward slash / with \/ which is unnecessary in .Net regular expressions. The regex language is the same but there are different tools which behave differently and php is not like .Net regex.
Secondly the star * on the ( ) is saying that there may be nothing in the parenthesis and your match is getting null nothing matches on every word.
Thirdly one does not need to split the whole word. I would just extract the commonality in the words into a set [ ]. That will allow the "or-ness" you need to match on either services or servicos. Such as
(/servic[oe]s)
Will inform you if services are found or not. Nothing else is needed.

Using Visual Studio 2013 regular expression find - how to invalidate multiple prefixes

I'm trying to find the "symbol" all in a bunch of C#, XML, and JS files. My project is huge and doing a naive search for "all" results in over 8,000 lines found so I'm trying to eliminate some of them.
For example, I don't want to match on "call" or "balloon" or "Balloon" (those are UX element styles.
Looking at the using Regular Expressions MSDN page (http://msdn.microsoft.com/en-us/library/vstudio/2k3te2cs%28v=vs.110%29.aspx) I found out how to invalidate on one of those but I can't figure out how to do it on multiple and make it case-insensitive.
I started off using:
(?!c)all
And that filtered out call and things like that but I can't get one to filter out multiple to work.
(?!b|c)all
Is the form I've been playing around with, trying to get it to ignore balloon. Ideally I could do something like (warning! - invalid regex below)
(?!b|c|B|C|)all
If anyone could point me in the right direction that would be great. The reason why I'm not looking for all surrounded by spaces is because I don't know if the reference I'm looking for is going to be:
.All
.all
("All")
(all)
and etc...

Have you tried: [aA][lL][lL]\b
any version of "all" or "ALL" anchored at a word/non-word boundary
Here is another reference..
Regular Expression to match specific string

The following regex: (?<!(b|c))all (with the IgnoreCase flag)
With the following input: ball all stall .all( "ALL"
Has the following matches: ball [all] st[all] .[all]( "[ALL]"

I think you are on the right track with lookarounds, but you want to use a character class with it. (More info: http://www.regular-expressions.info/charclass.html)
There are convenient shorthand character classes, like \w, to represent common classes. \w for example represents all alphanumeric characters and is shorthand for [A-Za-z0-9_].
\b represents a "word boundary," or in other words the beginning/end of the string and a boundary between a word character and a non-word character. It is zero-length and doesn't won't match any characters.
Here are some examples using a word boundary, positive lookaround, and negative lookaround respectively:
\b[aA][lL][lL]\b
(?<=[^\w])[aA][lL][lL](?=[^\w])
(?<!\w)[aA][lL][lL](?!\w)
Basically, these will find case-insensitive matches of "all" that are surrounded by non-alphanumeric characters. If you want to exclude certain surrounding characters, you can replace \w with your own character class (e.g. to exclude surrounding quote marks, use [A-Za-z0-9_"] instead of \w).

Noncapturing along with capturing match

I am trying to capture the subdomain from huge lists of domain names. For example I want to capture "funstuff" from "funstuff.mysite.com". I do not want to capture, ".mysite.com" in the match. These occurances are in a sea of text so I can not depend on them being at the start of a line. I know the subdomain will not include any special characters or numbers. So what I have is:
[a-z]{2,10}(?=\.mysite\.com)
The problem is this will work only if the subdomain is NOT preceded by a number or special character. For example, "asdfbasdasdfdfunstuff.mysite.com" will return "fdfunstuff" but "asdfasf23/funstuff.mysite.com" won't make a match.
I can not depend on there being a special character before the subdomain, like a "/" as in "http://funstuff.mysite.com" so that can not be used as part of the condition.
It is ok if the capture gets erroneous text before the subdomain, although 99% of the time it will be preceded with something other that a lowercase letter. I have tried,
(?<=[^a-z])[a-z]{2,10}(?=\.mysite\.com)
but for some reason this does not capture text is a situation like:
afb"asdfunstuff.mysite.com
Where the quotation mark prevents a match for [a-z]{2-20}. Basically what I would want to do in that case would be to capture asdfunstuff.mysite.com. How can this be accomplished?

So you've got two problems to solve: first, you want to match ".mysite.com" but not capture it; second, you want to grab up to 10 alphabetic characters in the "subdomain" position.
First problem can be solved by using a capturing group. The regex
([a-z]{2,10})\.mysite\.com
will capture somewhere between 2 and 10 characters, and the returned match object will expose that in one of its properties (depends on the language). C# returns a collection of Match objects, so it'll be the only item.
Second problem can be solved by using the word-boundary character \b. In .NET, this matches where an alphanumeric (i.e. \w) is next to a non-alphanumeric (\W). Other languages (e.g. ECMAScript / Javascript) work simliarly.
So, I suggest the following regex to solve your problem:
\b([a-z]{2,10})\.mysite\.com
Note that numbers are legal in subdomain names, too, so the following might be generally correct (though perhaps not in your specific case):
\b(\w{2,10})\.mysite\.com
where the "word character" \w is equivalent to [a-zA-Z_0-9] in .NET's ECMAScript-compliant mode. (Further reading.)

Character 'e' is not recognized by simple regular expression - why?

I wrote a very simple regular expression that need to match the next pattern:
word.otherWord
- Word must have at least 2 characters and must not start with digit.
I wrote the next expression:
[a-zA-Z][a-zA-Z](.[a-zA-Z0-9])+
I tested it using Regex tester and it seems to be working at most of the cases but when I try some inputs that ends with 'e' it's not working.
for example:
Hardware.Make does not work but Hardware.Makee is works fine, why? How can I fix it?

That's because your regex looks for inputs which length is even.
You have two characters matched by [a-zA-Z][a-zA-Z] and then another two characters matched by (.[a-zA-Z0-9]) as a group which is repeated one or more times (because of +).
You can see it here: http://regex101.com/r/fW2bC1
I think you need that:
[a-zA-Z]+(\.[a-zA-Z0-9]+)+

Actually, the dot is a regex metacharacter, which stands for "any character". You'll need to escape the dot.
For your situation, I'd do this:
[a-zA-Z]{2,}\.[a-zA-Z0-9]+
The {2,} means, at least 2 characters from the previous range.

In regex, the dot period is one of the most commonly used metacharacters and unfortunately also commonly misused metacharacter. The dot matches a single character without caring what that character is...
So u would also re-write it like
[a-zA-Z]+(\.[a-zA-Z0-9]+)+